The present invention relates to a liquid crystal display device. Particularly, the invention relates to a technique of preventing the invasion of foreign substance such as mote and dust into a backlight device which is disposed on a back side of a liquid crystal display panel and irradiates the liquid crystal display panel with light.
A conventional liquid crystal display device has a structure, as shown in FIG. 6, where a mold member MD formed of a resin member is disposed in a lower frame LF which is a metal member shaped like a box open on one side along a side wall and where the mold member MD retains a liquid crystal display panel PNL and a backlight device BU at predetermined positions. Particularly, the liquid crystal display device shown in FIG. 6 has the structure where a reflection sheet RF, a light guide LG, an optical sheet group OS and an unillustrated light source that constitute the backlight device BU are disposed on a bottom side of the mold member MD. On the other hand, the liquid crystal display panel PNL has a structure where an unillustrated liquid crystal is sealed between a first substrate SUB1 and a second substrate SUB2 and where polarizing plates PL1, PL2 are bonded to back and top sides thereof, respectively. The liquid crystal display panel PNL is mounted on an upper side of the mold member MD and is irradiated with a backlight beam from the backlight device BU. The liquid crystal display panel PNL and the backlight device BU are configured to be supported by a step part formed on an inner periphery of the mold member MD and an upper frame UF fitted on the lower frame UF so as to be retained at predetermined positions. Hence, the conventional liquid crystal display device has the structure, as shown in FIG. 6, where the upper frame UF is formed with an opening corresponding to a display area and a clearance is formed between the upper frame UF and the liquid crystal display panel PNL. Accordingly, the conventional liquid crystal display device has a problem that a foreign substance DS such as mote and dust intrudes through this clearance as indicated by an arrow ‘a’.
According to the structure of the conventional liquid crystal display device, the liquid crystal display panel PNL and the backlight device BU are retained at the predetermined positions by means of the step part formed at the mold member MD and the like. In conjunction with vibrations and the like of the liquid crystal display device, therefore, the foreign substance DS invading the liquid crystal display device intrudes further into the depth through clearance between the mold member MD and the liquid crystal display panel PNL, clearance between the mold member MD and the optical sheet group OS and the like, as indicated by arrows ‘b’, ‘c’. As a result, as shown in FIG. 6, the backlight device BU suffers the invasion of foreign substance DS in between the sheets constituting the optical sheet group OS, or between the light guide LG and the optical sheet group OS.
This invading foreign substance DS detrimentally intercepts planar light beam emitted from the light guide LG. Particularly in the case of the foreign substance DS intruding in between the optical sheets, the foreign substance DS is magnified (emphasized) so that a light portion intercepted by the foreign substance DS (intercepted light area) is projected on the liquid crystal display panel PNL as more magnified (emphasized). In a case where such a backlight beam containing the light portion intercepted by the foreign substance DS is made incident, the light portion intercepted by the foreign substance DS appears as a black spot in a display image. That is, the foreign substance causes a so-called black spot failure, display irregularities and the like, significantly lowering the quality (display grade) of the display image.
As the technique for preventing the invasion of foreign substance into the liquid crystal display device, there is known a technique set forth in, for example, JP-A No. 2004-272266 (Patent Literature 1) or U.S. Pat. No. 7,750,992 (Patent Literature 2) corresponding to the above. According to the technique set forth in Patent Literature 1 or Patent Literature 2, the display device has a structure which includes a first casing having a box-like shape open at the top, and a liquid-crystal display panel support member having a frame-like shape and disposed in the first casing in conformity with an inner periphery thereof. This liquid-crystal display panel support member is formed with a groove (engagement groove) and a step part at a portion conforming to the inner periphery of the first casing such that the engagement groove engages with a fixing part projected from the optical sheet while the step part supports the liquid crystal display panel. In this structure, a surface of the step part on which the liquid crystal display panel is mounted is formed with a belt-like projection for prevention of foreign-substance invention (first foreign-substance invasion preventing part) while a second foreign-substance invasion preventing part, which is formed independently from the liquid-crystal display panel support member, is disposed at a formation position of the engagement groove so as to prevent the invasion of foreign substance from the engagement groove.